Silicon nitride sintered bodies are widely used for wear-resistant members such as bearing balls, making the most of their high structural strength. Conventional silicon nitride sintered bodies used for the wear-resistant members can provide a higher strength, a three-point bending strength of 900 MPa or more, while they have a low thermal conductivity of 20 W/m·K or less.
Examples of applications that have made the most of the high strength properties of silicon nitride sintered bodies in recent years include substrates for semiconductors. In Japanese Patent No. 3797905 (Patent Document 1), there is disclosed a silicon nitride substrate comprising a silicon nitride sintered body having a thermal conductivity of 50 W/m·K or more and a three-point bending strength of 500 MPa or more. In Patent Document 1, it is described that by setting the maximum pore diameter in the grain boundary phases at 0.3 μm or less, the current leakage value can be decreased to 1000 nA or less even if the thickness of the silicon nitride substrate is 0.3 to 1.5 mm.
A current leakage value represents the value of a current that flows between the front and back of a silicon nitride substrate when an alternating voltage under certain conditions is applied. As this current leakage value becomes smaller, higher insulating properties of the silicon nitride substrate are indicated. When the insulating properties of a silicon nitride substrate are low, conduction occurs between the front and the back of the silicon nitride substrate, thus resulting in the cause of the misoperation (mal-functioning) of a semiconductor apparatus.
Good heat dissipation (radiation) properties are required for a semiconductor substrate. Semiconductor devices include various ones such as Si semiconductors and SiC semiconductors. With higher output of semiconductor devices, the junction temperature increases from about 100° C. to about 200° C. Examples of measures for improving the heat dissipation properties of a semiconductor substrate include increasing the thermal conductivity of a silicon nitride sintered body and thinning a silicon nitride substrate. Particularly when a silicon nitride substrate can be thinned, smaller size and lighter weight of a semiconductor apparatus can also be actually achieved. At this time, with different materials such as ceramic and a metal electrode, strain stress occurs in the bonding portion because of the difference between their respective thermal expansion rates, and the like. Therefore, a certain level or more of material strength is required for a silicon nitride substrate, and the improvement of mechanical strength is also required.